Summary: Behavioral/Systems/Cognitive
Higher-Dimensional Neurons Explain the Tuning and
Dynamics of Working Memory Cells
Ray Singh1 and Chris Eliasmith1,2
Departments of 1Systems Design Engineering and 2Philosophy, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Measurements of neural activity in working memory during a somatosensory discrimination task show that the content of working
memory is not only stimulus dependent but also strongly time varying. We present a biologically plausible neural model that reproduces
the wide variety of characteristic responses observed in those experiments. Central to our model is a heterogeneous ensemble of two-
dimensional neurons that are hypothesized to simultaneously encode two distinct stimuli dimensions. We demonstrate that the spiking
activity of each neuron in the population can be understood as the result of a two-dimensional state space trajectory projected onto the
tuningcurveoftheneuron.Thewidevarietyofobservedresponsesisthusanaturalconsequenceofapopulationofneuronswithadiverse
set of preferred stimulus vectors and response functions in this two-dimensional space. In addition, we propose a taxonomy of network
topologies that will generate the two-dimensional trajectory necessary to exploit this population. We conclude by proposing some
experimental indicators to help distinguish among these possibilities.
Key words: computational model; neural dynamics; population coding; reward uncertainty; working memory; cognitive
Introduction
The majority of work related to working memory takes stably
persistent activity to be an indicator that a cell is participating in
remembering a stimulus (Fuster, 1973; Gnadt and Andersen,
1988; Funahashi et al., 1989; Zhang, 1999; Taube and Basset,